Sensor element

ABSTRACT

A heater part of a planar sensor element includes: a heater element containing Pt; an insulating layer covering the heater element; and a heater electrode located on a main surface of the sensor element to be exposed. A portion of the heater part other than the heater electrode is buried in a base part formed of a solid electrolyte. The insulating layer at least contains MgO, MgAl2O4, and Mg4Nb2O9 in a weight percentage of 97 wt % to 100 wt % in total. MgO, MgAl2O4, and Mg4Nb2O9 are contained in weight percentages of 30 wt % to 60 wt %, 30 wt % to 60 wt %, and 0.5 wt % to 15 wt %, respectively. The insulating layer has a porosity of 4% or less.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to a sensor element formed of an oxygen-ion conductive solid electrolyte, and, in particular, to the configuration of a heater included in the sensor element.

Description of the Background Art

As an apparatus for measuring the concentration of a predetermined gas component (e.g., O₂, NOx, HC, and CO) in a measurement gas, including a combustion gas and an exhaust gas in an internal combustion engine, such as an engine of a vehicle, a gas sensor including a sensor element formed of an oxygen-ion conductive solid electrolyte, such as zirconia (ZrO₂), has been known.

As such a sensor element of the gas sensor (gas sensor element), a sensor element having a structure in which a sensor part for performing detection of a target gas component and the like and a heater part including a heater for heating the sensor part to activate the oxygen-ion conductive solid electrolyte forming the sensor part are laminated and integrated is widely known. The sensor element having this structure has the advantage of being able to quickly heat the sensor element (sensor part) to a desired driving temperature (or activation temperature) at the start of driving of the gas sensor.

Such a heater usually includes a heater element including a resistive heating part formed of metal, such as Pt (platinum), and an insulating layer surrounding the heater element to electrically insulate the heater element from the surroundings. Alumina, spinel, or the like is used as an insulating material for the insulating layer.

When such a sensor element is used continuously, a temperature cycle including heating during use and cooling after use is repeated, and, each time the temperature cycle is repeated, stress is caused on the insulating layer by a difference in coefficient of thermal expansion between the insulating layer and solid electrolyte layers surrounding the insulating layer. A sensor element in which the insulating layer has a porous structure to prevent or reduce the occurrence of cracking in the insulating layer caused by the action of the stress is already known (see, for example, Japanese Patent No. 3668050).

A sensor element in which the coefficient of thermal expansion (thermal expansion rate) of the solid electrolyte layers or the insulating layer has been adjusted to relax the stress caused due to the difference in thermal expansion is also already known (see, for example, Japanese Patent No. 3873302 and Japanese Patent No. 4980996). Japanese Patent No. 3873302 discloses an embodiment of adjusting the coefficient of thermal expansion of the solid electrolyte layers by causing zirconia forming the solid electrolyte layers to be in mixed phase of C phase (cubic phase) and M phase (monoclinic phase). On the other hand, Japanese Patent No. 4980996 discloses an embodiment of adjusting the coefficient of thermal expansion of the insulating layer by adding a rare-earth element to alumina forming the insulating layer, so that the coefficient of thermal expansion of the insulating layer is equal to the coefficient of thermal expansion of the solid electrolyte layers.

Furthermore, a sensor element in which only an interface portion of the insulating layer with the solid electrolyte layers being in contact with the insulating layer has been densified to ensure adhesion of the solid electrolyte layers to the insulating layer is already known (see, for example, Japanese Patent No. 3096281).

In addition, a cylindrical sensor element in which reduction in thermal shock resistance is prevented or reduced while including a dense insulating layer is already known (see, for example, Japanese Patent No. 4573939).

The driving temperature (heating temperature to which the sensor element is heated by the heater) when the sensor element is driven may be determined appropriately in accordance with the configuration of the sensor element, a material for each component, performance requirements for the sensor element, and the like. However, in the case that a conventional sensor element including a heater that includes a heater element formed of Pt and a porous insulating layer is used continuously while setting the driving temperature to a relatively high temperature of approximately 850° C., a problem in that a heater resistance value increases over time to eventually disable the sensor element is more likely to occur compared with a case where the driving temperature is low.

It is assumed, from intensive studies made by the inventors of the present invention, that the increase in heater resistance value is caused because Pt, which forms the heater element heated to a high temperature at the time of driving, changes into a gas phase, and diffuses in the porous insulating layer.

The configuration to prevent or reduce such diffusion of Pt during use is required to increase a sensor element lifetime, while the occurrence of cracking caused by the difference in coefficient of thermal expansion between the solid electrolyte layers and the insulating layer is required to be prevented or reduced, as required in the conventional sensor element.

Japanese Patent No. 3668050, Japanese Patent No. 3873302, Japanese Patent No. 4980996, Japanese Patent No. 3096281, and Japanese Patent No. 4573939 each neither disclose nor suggest prevention or reduction of diffusion of Pt when the sensor element is used.

SUMMARY

The present invention is directed to a sensor element formed of an oxygen-ion conductive solid electrolyte, and relates, in particular, to the configuration of a heater included in the sensor element.

According to the present invention, a planar sensor element included in a gas sensor for detecting a predetermined gas component in a measurement gas includes: a base part that is formed of zirconia as an oxygen-ion conductive solid electrolyte; and a heater part that heats the sensor element. The heater part includes: a heater element that generates heat by being externally powered, and contains Pt; an insulating layer that covers the heater element; and a heater electrode that is located on a main surface of the sensor element to be exposed, and is electrically connected to the heater element. A portion of the heater part other than the heater electrode is buried in the base part. The insulating layer at least contains MgO, MgAl₂O₄, and Mg₄Nb₂O₉ in a weight percentage of 97 wt % to 100 wt % in total. MgO is contained in a weight percentage of 30 wt % to 60 wt %, MgAl₂O₄ is contained in a weight percentage of 30 wt % to 60 wt %, and Mg₄Nb₂O₉ is contained in a weight percentage of 0.5 wt % to 15 wt %. The insulating layer has a porosity of 4% or less.

According to the present invention, when the sensor element is used continuously, and a temperature cycle including heating during use and cooling after use is repeated, diffusion of Pt and the occurrence of cracking caused by a difference in coefficient of thermal expansion between solid electrolyte layers and a heater insulating layer are suitably prevented or reduced, enabling the planar sensor element to have an increased lifetime.

An object of the present invention is thus to provide a sensor element having an increased lifetime due to prevention or reduction of diffusion of Pt during use and prevention or reduction of the occurrence of cracking caused by a difference in coefficient of thermal expansion between solid electrolyte layers and an insulating layer.

These and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically shows an example of the configuration of a gas sensor 100 including a vertical sectional view taken along the longitudinal direction of a sensor element 101;

FIG. 2 is a schematic view of a cross section perpendicular to the longitudinal direction of the sensor element 101 taken along the line A-A′ of FIG. 1;

FIG. 3 shows a schematic planar arrangement of principal components of a heater part 70;

FIG. 4 is a flowchart showing processing performed when the sensor element 101 is manufactured; and

FIG. 5 shows procedures for forming a heater element 72 and a heater insulating layer 74 in more details.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

<Schematic Configuration of Gas Sensor>

A schematic configuration of a gas sensor 100 including a sensor element 101 according to the present embodiment will be described first. Description is made in the present embodiment by taking, as an example, a case where the gas sensor 100 is a limiting current type NOx sensor that detects NOx and measures the concentration of NOx using the sensor element 101.

FIG. 1 schematically shows an example of the configuration of the gas sensor 100 including a vertical sectional view taken along the longitudinal direction of the sensor element 101. FIG. 2 is a schematic view of a cross section perpendicular to the longitudinal direction of the sensor element 101 taken along the line A-A′ of FIG. 1.

The sensor element 101 is a planar (an elongated plate-shaped) element having a structure in which six solid electrolyte layers, namely, a first substrate layer 1, a second substrate layer 2, a third substrate layer 3, a first solid electrolyte layer 4, a spacer layer 5, and a second solid electrolyte layer 6, each formed of zirconia (ZrO₂) (e.g., yttrium stabilized zirconia (YSZ)) being an oxygen-ion conductive solid electrolyte, are laminated in the stated order from the bottom side of FIG. 1. A solid electrolyte forming these six layers is dense and airtight. An upper surface and a lower surface of each of these six layers in FIG. 1 are hereinafter also simply referred to as an upper surface and a lower surface, respectively. A part of the sensor element 101 formed of a solid electrolyte as a whole is generically referred to as a base part.

The sensor element 101 is manufactured, for example, by performing predetermined machining and printing of circuit patterns with respect to ceramic green sheets corresponding to respective layers, then laminating these green sheets, and further firing the laminated green sheets for integration.

Between a lower surface of the second solid electrolyte layer 6 and an upper surface of the first solid electrolyte layer 4 at one end portion of the sensor element 101, a gas inlet 10, a first diffusion control part 11, a buffer space 12, a second diffusion control part 13, a first internal space 20, a third diffusion control part 30, and a second internal space 40 are formed adjacent to each other to communicate in the stated order.

The gas inlet 10, the buffer space 12, the first internal space 20, and the second internal space 40 are spaces inside the sensor element 101 that look as if they were provided by hollowing out the spacer layer 5, and that have an upper portion, a lower portion, and a side portion respectively defined by the lower surface of the second solid electrolyte layer 6, the upper surface of the first solid electrolyte layer 4, and a side surface of the spacer layer 5.

The first diffusion control part 11, the second diffusion control part 13, and the third diffusion control part 30 are each provided as two horizontally long slits (whose openings have longitudinal directions perpendicular to the page of FIG. 1). A part extending from the gas inlet 10 to the second internal space 40 is also referred to as a gas distribution part.

At a location farther from the end portion than the gas distribution part is, a reference gas introduction space 43 having a side portion defined by a side surface of the first solid electrolyte layer 4 is provided between an upper surface of the third substrate layer 3 and a lower surface of the spacer layer 5. For example, atmospheric air is introduced into the reference gas introduction space 43 as a reference gas when a NOx concentration is measured.

An atmospheric air introduction layer 48 is a layer formed of porous alumina, and the reference gas is introduced into the atmospheric air introduction layer 48 through the reference gas introduction space 43. The atmospheric air introduction layer 48 is formed to cover a reference electrode 42.

The reference electrode 42 is an electrode formed to be sandwiched between the upper surface of the third substrate layer 3 and the first solid electrolyte layer 4, and the atmospheric air introduction layer 48 leading to the reference gas introduction space 43 is provided around the reference electrode 42, as described above. As will be described below, an oxygen concentration (oxygen partial pressure) in the first internal space 20 and the second internal space 40 can be measured using the reference electrode 42.

In the gas distribution part, the gas inlet 10 opens to an external space, and a measurement gas is taken from the external space into the sensor element 101 through the gas inlet 10.

The first diffusion control part 11 is a part providing a predetermined diffusion resistance to the measurement gas taken through the gas inlet 10.

The buffer space 12 is a space provided to guide the measurement gas introduced from the first diffusion control part 11 to the second diffusion control part 13.

The second diffusion control part 13 is a part providing a predetermined diffusion resistance to the measurement gas introduced from the buffer space 12 into the first internal space 20.

When the measurement gas is introduced from the outside of the sensor element 101 into the first internal space 20, the measurement gas, which is abruptly taken into the sensor element 101 through the gas inlet 10 due to pressure fluctuation of the measurement gas in the external space (pulsation of exhaust pressure in a case where the measurement gas is an exhaust gas of a vehicle), is not directly introduced into the first internal space 20, but is introduced into the first internal space 20 after the concentration fluctuation of the measurement gas is canceled through the first diffusion control part 11, the buffer space 12, and the second diffusion control part 13. This makes the concentration fluctuation of the measurement gas introduced into the first internal space 20 almost negligible.

The first internal space 20 is provided as a space used to adjust oxygen partial pressure in the measurement gas introduced through the second diffusion control part 13. The oxygen partial pressure is adjusted by operation of a main pump cell 21.

The main pump cell 21 is an electrochemical pump cell constituted by an inner pump electrode 22, an outer pump electrode 23, and the second solid electrolyte layer 6 sandwiched between the inner pump electrode 22 and the outer pump electrode 23. The inner pump electrode 22 has a ceiling electrode portion 22 a that is provided substantially on the entire lower surface of a portion of the second solid electrolyte layer 6 facing the first internal space 20. The outer pump electrode 23 is provided in a region, on an upper surface of the second solid electrolyte layer 6 (one main surface of the sensor element 101), corresponding to the ceiling electrode portion 22 a so as to be exposed to the external space.

The inner pump electrode 22 is formed over upper and lower solid electrolyte layers (the second solid electrolyte layer 6 and the first solid electrolyte layer 4) that define the first internal space 20, and the spacer layer 5 that provides a side wall to the first internal space 20. Specifically, the ceiling electrode portion 22 a is formed on the lower surface of the second solid electrolyte layer 6, which provides a ceiling surface to the first internal space 20, a bottom electrode portion 22 b is formed on the upper surface of the first solid electrolyte layer 4, which provides a bottom surface to the first internal space 20, and a side electrode portion 22 c (FIG. 2) is formed on a side wall surface (internal surface) of the spacer layer 5 that forms opposite side wall portions of the first internal space 20, so as to connect the ceiling electrode portion 22 a and the bottom electrode portion 22 b. The inner pump electrode 22 is thus provided in the form of a tunnel at a location where the side electrode portion is provided.

The inner pump electrode 22 and the outer pump electrode 23 are each formed as a porous cermet electrode (e.g., a cermet electrode formed of ZrO₂ and Pt that contains Au of 1%). The inner pump electrode 22 to be in contact with the measurement gas is formed using a material having a weakened reducing ability with respect to a NOx component in the measurement gas.

The main pump cell 21 can pump out oxygen in the first internal space 20 to the external space or pump in oxygen in the external space to the first internal space 20 by applying, using a variable power supply 24, a desired pump voltage Vp0 across the inner pump electrode 22 and the outer pump electrode 23 to allow a pump current Ip0 to flow between the inner pump electrode 22 and the outer pump electrode 23 in a positive or negative direction.

To detect an oxygen concentration (oxygen partial pressure) in the atmosphere existing in the first internal space 20, the inner pump electrode 22, the second solid electrolyte layer 6, the spacer layer 5, the first solid electrolyte layer 4, the third substrate layer 3, and the reference electrode 42 constitute an electrochemical sensor cell, namely, a main-pump-control oxygen-partial-pressure detection sensor cell 80.

The oxygen concentration (oxygen partial pressure) in the first internal space 20 can be obtained by measuring electromotive force V0 in the main-pump-control oxygen-partial-pressure detection sensor cell 80.

Furthermore, the pump current Ip0 is controlled by performing feedback control of the voltage Vp0 so that the electromotive force V0 is maintained constant. The oxygen concentration in the first internal space 20 is thereby maintained to have a predetermined constant value.

The third diffusion control part 30 is a part providing a predetermined diffusion resistance to the measurement gas having an oxygen concentration (oxygen partial pressure) controlled by the operation of the main pump cell 21 in the first internal space 20, and guiding the measurement gas to the second internal space 40.

The second internal space 40 is provided as a space to perform processing concerning measurement of a nitrogen oxide (NOx) concentration in the measurement gas introduced through the third diffusion control part 30. The NOx concentration is measured, mainly in the second internal space 40 in which an oxygen concentration has been adjusted by an auxiliary pump cell 50, by the operation of a measurement pump cell 41.

After the oxygen concentration (oxygen partial pressure) is adjusted in advance in the first internal space 20, the auxiliary pump cell 50 further adjusts the oxygen partial pressure of the measurement gas introduced through the third diffusion control part in the second internal space 40. Owing to such adjustment, the oxygen concentration in the second internal space 40 can be maintained constant with high precision, and thus the gas sensor 100 is enabled to measure the NOx concentration with high precision.

The auxiliary pump cell 50 is an auxiliary electrochemical pump cell constituted by an auxiliary pump electrode 51, the outer pump electrode 23 (not limited to the outer pump electrode 23 but may be any appropriate electrode outside the sensor element 101), and the second solid electrolyte layer 6. The auxiliary pump electrode 51 has a ceiling electrode portion 51 a that is provided substantially on the entire lower surface of a portion of the second solid electrolyte layer 6 facing the second internal space 40.

The auxiliary pump electrode 51 is provided in the second internal space 40 in the form of a tunnel, as with the inner pump electrode 22 provided in the first internal space 20 described previously. That is to say, the ceiling electrode portion 51 a is formed on the second solid electrolyte layer 6, which provides a ceiling surface to the second internal space 40, a bottom electrode portion 51 b is formed on the first solid electrolyte layer 4, which provides a bottom surface to the second internal space 40, and a side electrode portion (not illustrated) that connects the ceiling electrode portion 51 a and the bottom electrode portion 51 b is formed on opposite wall surfaces of the spacer layer 5, which provides a side wall to the second internal space 40. The auxiliary pump electrode 51 is thus provided in the form of a tunnel.

As with the inner pump electrode 22, the auxiliary pump electrode 51 is formed using a material having a weakened reducing ability with respect to a NOx component in the measurement gas.

The auxiliary pump cell 50 can pump out oxygen in the atmosphere existing in the second internal space 40 to the external space or pump in oxygen existing in the external space to the second internal space 40 by applying a desired voltage Vp1 across the auxiliary pump electrode 51 and the outer pump electrode 23.

In order to control the oxygen partial pressure in the atmosphere in the second internal space 40, the auxiliary pump electrode 51, the reference electrode 42, the second solid electrolyte layer 6, the spacer layer 5, the first solid electrolyte layer 4, and the third substrate layer 3 constitute an electrochemical sensor cell, namely, an auxiliary-pump-control oxygen-partial-pressure detection sensor cell 81.

The auxiliary pump cell 50 performs pumping using a variable power supply 52 whose voltage is controlled based on electromotive force V1 detected by the auxiliary-pump-control oxygen-partial-pressure detection sensor cell 81. The oxygen partial pressure in the atmosphere in the second internal space 40 is thereby controlled to a low partial pressure having substantially no effect on measurement of NOx.

At the same time, a resulting pump current Ip1 is used to control electromotive force in the main-pump-control oxygen-partial-pressure detection sensor cell 80. Specifically, the pump current Ip1 is input, as a control signal, into the main-pump-control oxygen-partial-pressure detection sensor cell 80, and, through control of the electromotive force V0 thereof, the oxygen partial pressure in the measurement gas introduced through the third diffusion control part 30 into the second internal space 40 is controlled to have a gradient that is always constant. In use as a NOx sensor, the oxygen concentration in the second internal space 40 is maintained to have a constant value of approximately 0.001 ppm by the action of the main pump cell 21 and the auxiliary pump cell 50.

The measurement pump cell 41 measures the NOx concentration in the measurement gas in the second internal space 40. The measurement pump cell 41 is an electrochemical pump cell constituted by a measurement electrode 44, the outer pump electrode 23, the second solid electrolyte layer 6, the spacer layer 5, and the first solid electrolyte layer 4. The measurement electrode 44 is provided on an upper surface of a portion of the first solid electrolyte layer 4 facing the second internal space 40 to be separated from the third diffusion control part 30.

The measurement electrode 44 is a porous cermet electrode. The measurement electrode 44 also functions as a NOx reduction catalyst that reduces NOx existing in the atmosphere in the second internal space 40. Furthermore, the measurement electrode 44 is covered with a fourth diffusion control part 45.

The fourth diffusion control part 45 is a film formed of a porous body containing alumina (Al₂O₃) as a main component. The fourth diffusion control part 45 plays a role in limiting the amount of NOx flowing into the measurement electrode 44, and also functions as a protective film of the measurement electrode 44.

The measurement pump cell 41 can pump out oxygen generated through decomposition of nitrogen oxides in the atmosphere around the measurement electrode 44, and detect the amount of generated oxygen as a pump current Ip2.

In order to detect the oxygen partial pressure around the measurement electrode 44, the second solid electrolyte layer 6, the spacer layer 5, the first solid electrolyte layer 4, the third substrate layer 3, the measurement electrode 44, and the reference electrode 42 constitute an electrochemical sensor cell, namely, a measurement-pump-control oxygen-partial-pressure detection sensor cell 82. A variable power supply 46 is controlled based on electromotive force V2 detected by the measurement-pump-control oxygen-partial-pressure detection sensor cell 82.

The measurement gas introduced into the second internal space 40 reaches the measurement electrode 44 through the fourth diffusion control part 45 under a condition in which the oxygen partial pressure is controlled. Nitrogen oxides in the measurement gas around the measurement electrode 44 are reduced (2NO→N₂+O₂) to generate oxygen. The generated oxygen is pumped by the measurement pump cell 41, and, at that time, a voltage Vp2 of the variable power supply 46 is controlled so that a control voltage V2 detected by the measurement-pump-control oxygen-partial-pressure detection sensor cell 82 is maintained constant. The amount of oxygen generated around the measurement electrode 44 is proportional to a nitrogen oxide concentration in the measurement gas, and thus the nitrogen oxide concentration in the measurement gas is calculated using the pump current Ip2 in the measurement pump cell 41.

If the measurement electrode 44, the first solid electrolyte layer 4, the third substrate layer 3, and the reference electrode 42 are combined to constitute an oxygen partial pressure detection means as an electrochemical sensor cell, electromotive force in accordance with a difference between the amount of oxygen generated through reduction of a NOx component in the atmosphere around the measurement electrode 44 and the amount of oxygen contained in reference atmospheric air can be detected, and the concentration of the NOx component in the measurement gas can thereby be obtained.

The second solid electrolyte layer 6, the spacer layer 5, the first solid electrolyte layer 4, the third substrate layer 3, the outer pump electrode 23, and the reference electrode 42 constitute an electrochemical sensor cell 83, and oxygen partial pressure in the measurement gas outside the sensor can be detected using electromotive force Vref obtained by the sensor cell 83.

In the gas sensor 100 having such a configuration, the measurement gas in which the oxygen partial pressure is always maintained to have a constant low value (value having substantially no effect on measurement of NOx) by operation of the main pump cell 21 and the auxiliary pump cell 50 is provided to the measurement pump cell 41. The NOx concentration in the measurement gas can be known based on the fact that the pump current Ip2 flowing by the measurement pump cell 41 pumping out oxygen generated through reduction of NOx performed by the measurement electrode 44 is approximately proportional to the concentration of NOx in the measurement gas.

<Heater Part>

The sensor element 101 further includes a heater part 70 playing a role in temperature adjustment of heating the sensor element 101 and keeping it warm, in order to enhance the oxygen ion conductivity of the solid electrolyte forming the base part. FIG. 3 shows a schematic planar arrangement of principal components of the heater part 70.

The heater part 70 includes heater electrodes 71 (71 a, 71 b, and 71 c), a heater element 72, heater leads 72 a (72 a 1 and 72 a 2), a resistance detection lead 72 b, through holes 73, and a heater insulating layer 74. In FIG. 3, however, the through holes 73 and the heater insulating layer 74 are omitted. A portion of the heater part 70 other than the heater electrodes 71 is buried in the base part of the sensor element 101.

The heater electrodes 71 (71 a, 71 b, and 71 c) are electrodes formed to be in contact with a lower surface of the first substrate layer 1 (the other main surface of the sensor element 101).

The heater element 72 is a resistive heating element located between the second substrate layer 2 and the third substrate layer 3. The heater element 72 generates heat by being powered from the outside of the sensor element 101 through the heater electrodes 71, the through holes 73, and the heater leads 72 a, which constitute a current-carrying path. The heater element 72 is formed of Pt, or formed to contain Pt as a main component. The heater element 72 is buried, in a predetermined range of the sensor element 101 in which the gas distribution part is provided, opposite to the gas distribution part in a direction of the thickness of the element. The heater element 72 is provided to have a thickness of approximately 10 μm to 20 μm.

A pair of heater leads (the heater leads 72 a 1 and 72 a 2) connected to the respective ends of the heater element 72 is provided to have approximately the same shape, that is, to have the same resistance value. The heater leads 72 a 1 and 72 a 2 are respectively connected to the heater electrodes 71 a and 71 b through corresponding through holes 73.

Furthermore, the resistance detection lead 72 b is provided to be drawn from a connection portion 75 at which the heater element 72 and the heater lead 72 a 2 are connected. Assume that a resistance value of the resistance detection lead 72 b is negligible. The resistance detection lead 72 b is connected to the heater electrode 71 c through a corresponding through hole 73.

In the sensor element 101, each component of the sensor element 101 can be heated to a predetermined temperature and kept warm by allowing a current to flow between the heater electrodes 71 a and 71 b to perform heating using the heater element 72. Specifically, the sensor element 101 is heated so that the solid electrolyte around the gas distribution part is at a temperature of approximately 750° C. to 950° C. The oxygen ion conductivity of the solid electrolyte forming the base part in the sensor element 101 is enhanced by the heating.

Since the heater leads 72 a 1 and 72 a 2 have the same resistance value, and the resistance value of the resistance detection lead 72 b is negligible, when R₁ denotes the resistance value between the heater electrodes 71 a and 71 b, and R₂ denotes the resistance value between the heater electrodes 71 b and 71 c, the resistance value (heater resistance value) R_(H) of the heater element 72 is calculated by the following equation: R _(H) =R ₁ −R ₂  (1)

The heater resistance value is used to control the heating temperature when heating is performed using the heater element 72.

The above-mentioned thickness of the heater element 72 is determined so that the heater resistance value becomes a value within a predetermined range determined from the viewpoint of performance, a lifetime, and the like of the sensor element 101.

The heater insulating layer 74 is an insulating layer formed to cover the heater element 72. The heater insulating layer 74 is formed for electrical insulation between the second substrate layer 2 and the heater element 72 and for electrical insulation between the third substrate layer 3 and the heater element 72. The heater insulating layer 74 has a thickness of approximately 70 μm to 110 μm, and is located to be separated from end surfaces and side surfaces of the sensor element 101 by approximately 200 μm to 700 μm. The thickness of the heater insulating layer 74, however, may not necessarily be constant, and may differ at a location at which the heater element 72 exists and at a location at which the heater element 72 does not exist.

The heater insulating layer 74 contains, as main components, MgO, MgAl₂O₄ (spinel), and Mg₄Nb₂O₉ in a weight percentage of 97 wt % to 100 wt % in total. MgO and MgAl₂O₄ are each contained in a weight percentage of 30 wt % to 60 wt %. Mg₄Nb₂O₉ is contained in a weight percentage of 0.5 wt % to 15 wt %. These requirements relating to the weight percentages of the main components of the heater insulating layer 74 are hereinafter referred to as main component weight percentage conditions. MgO and MgAl₂O₄ are not required to be contained in the same weight percentage. In the present embodiment, the heater insulating layer 74 satisfying the main component weight percentage conditions is provided as a densified layer having a porosity of 4.0% or less.

The heater insulating layer 74 also contains, as sub components, a compound of Si and a compound of Ba respectively derived from SiO₂ and BaCO₃ contained in a raw powder as sintering aids.

The porosity of the heater insulating layer 74 of 4.0% or less is achieved by mixing materials for heater insulating layer formation so that the heater insulating layer 74 satisfies the above-mentioned main component weight percentage conditions, and by setting firing conditions when the sensor element 101 is eventually obtained to specific conditions. Details will be described below.

In the present embodiment, assume that the porosity of the heater insulating layer 74 is calculated by capturing, using an SEM, an image of a cross section of the sensor element 101, including the heater element 72, perpendicular to the longitudinal direction of the sensor element 101 after polishing, and binarizing an SEM image as obtained.

The reason that the heater insulating layer 74 is provided as the densified layer is to prevent diffusion of Pt, which forms the heater element 72, into pores of the heater insulating layer 74 when the sensor element 101 is used. In the sensor element 101 according to the present embodiment, the heater insulating layer 74 is provided as the densified layer having a porosity of 4.0% or less, so that damage to the heater element 72 caused by diffusion of Pt is suitably prevented or reduced even if the sensor element 101 is used continuously. The sensor element thus has an increased lifetime, compared with the conventional sensor element.

Whether Pt diffuses from the heater element 72 or not can directly be determined by actually exposing the heater insulating layer 74 of the sensor element 101 after use for a given period of time, and observing an image and analyzing the composition of the exposed heater insulating layer using the SEM and the like, and also can be known from an increase rate of a heater resistance value R (resistance increase rate) after use when a heater resistance value R0 before use (in an initial stage of use) is used as a reference, which is calculated by the following equation (2): resistance increase rate (%)=100×(R−R0)/R0  (2)

The heater resistance value is an electrical resistance value on a path of a heater current from one of the heater electrodes 71 to the other one of the heater electrodes 71, and it is confirmed in advance that the heater resistance value can vary mainly due to diffusion of Pt from the heater element 72. Thus, by measuring the heater resistance value both in the initial stage of use and after use, and determining whether the resistance increase rate calculated by the equation (2) exceeds a predetermined threshold or not, whether Pt diffuses from the heater element 72 or not can be determined using the sensor element 101 without breaking the sensor element 101.

Specifically, it is determined that Pt diffuses from the heater element 72 when the resistance increase rate is 2% or more. In the sensor element 101 according to the present embodiment, in which the heater insulating layer 74 has a porosity of 4.0% or less, the resistance increase rate calculated by the equation (2) is limited to less than 2%.

In addition, in the sensor element 101 according to the present embodiment, the coefficient of thermal expansion (coefficient of linear expansion) of the heater insulating layer 74 at least at 900° C. is in a range of 10.3 to 12.3 (×10⁻⁶/° C.). The value of the coefficient of thermal expansion is approximately equal to the coefficient of thermal expansion of zirconia (approximately 10 to 11 (×10⁻⁶/° C.)) forming the solid electrolyte layers in the sensor element 101. This is also achieved by mixing the materials for heater insulating layer formation so that the heater insulating layer 74 satisfies the main component weight percentage conditions. The coefficient of thermal expansion of the heater insulating layer 74 tends to increase with increasing weight percentage of MgO relative to MgAl₂O₄ and to decrease with decreasing weight percentage of MgO relative to MgAl₂O₄.

That is to say, in the sensor element 101 according to the present embodiment, the heater insulating layer 74 is provided as the densified layer having a coefficient of thermal expansion approximately equal to that of the surrounding solid electrolyte layers (e.g., the second substrate layer 2 and the third substrate layer 3).

In the present embodiment, assume that the coefficient of thermal expansion of the heater insulating layer 74 is obtained by a thermomechanical analyzer (TMA) using a portion corresponding to the heater insulating layer 74 cut out of the sensor element 101 as a test sample.

As mentioned in Japanese Patent No. 4573939, conventionally, when the heater insulating layer was provided as the densified layer using, as a main component material for the heater insulating layer, alumina (coefficient of thermal expansion: approximately 7 to 9 (×10⁻⁶/° C.)) typically used as an insulating material, in a planar sensor element similar to the sensor element 101 according to the present embodiment, there were some cases where cracking caused by a difference in coefficient of thermal expansion between the heater insulating layer and the solid electrolyte layers occurred as a result of repeating a temperature cycle including heating during use and cooling after use.

In contrast, in the sensor element 101 according to the present embodiment, the coefficient of thermal expansion of the heater insulating layer 74 has been adjusted to have a value in the above-mentioned range, so that the difference in coefficient of thermal expansion between the heater insulating layer 74 and the solid electrolyte layers (in particular, the second substrate layer 2 and the third substrate layer 3) is small. Thus, although the sensor element 101 is planar and includes the densified heater insulating layer 74, the occurrence of cracking caused by the difference in coefficient of thermal expansion can suitably be prevented or reduced. It can thus be said that the sensor element 101 according to the present embodiment has an increased lifetime also from the viewpoint of prevention or reduction of the occurrence of cracking in addition to prevention or reduction of diffusion of Pt.

<Process of Manufacturing Sensor Element>

The process of manufacturing the sensor element 101 having a configuration and features as described above will be described next. In the present embodiment, the sensor element 101 is manufactured by forming a laminated body formed of green sheets containing an oxygen-ion conductive solid electrolyte, such as zirconia, as a ceramic component, and by cutting and firing the laminated body.

A case where the sensor element 101 including the six layers illustrated in FIG. 1 is manufactured will be described, as an example, below. In this case, six green sheets corresponding to the first substrate layer 1, the second substrate layer 2, the third substrate layer 3, the first solid electrolyte layer 4, the spacer layer 5, and the second solid electrolyte layer 6 are prepared.

FIG. 4 is a flowchart showing processing performed when the sensor element 101 is manufactured. FIG. 5 shows procedures for forming the heater element 72 and the heater insulating layer 74 in more details.

In the manufacture of the sensor element 101, blank sheets (not illustrated) that are green sheets having no pattern formed thereon are prepared first (step S1). In the manufacture of the sensor element 101 including the six layers, six blank sheets corresponding to the respective layers are prepared. The blank sheets have a plurality of sheet holes used for positioning in printing and lamination. The sheet holes are formed in advance through, for example, punching by a punching machine when the sheets for the respective layers are in the form of the blank sheets prior to pattern formation. Green sheets corresponding to layers forming an internal space also include penetrating portions corresponding to the internal space formed in advance through, for example, punching as described above. The blank sheets corresponding to the respective layers of the sensor element 101 are not required to have the same thickness.

After preparation of the blank sheets corresponding to the respective layers, pattern printing and drying are performed on the individual blank sheets (step S2). A known screen printing technique can be used for printing of patterns and adhesives. A known drying means can be used for drying after printing.

As for formation of a pattern (hereinafter, a heater pattern) that becomes the heater element 72, the heater leads 72 a, and the resistance detection lead 72 b and a pattern that becomes the heater insulating layer 74, for example, a blank sheet that becomes the second substrate layer 2 after firing is prepared first as a target of printing, as shown in FIG. 5 (step S21). Considering deformation of the blank sheet and the like caused by repetition of printing, a laminate (preceding laminated sheet) of the blank sheet that becomes the first substrate layer 1 and the blank sheet that becomes the second substrate layer 2 may be prepared in advance.

When the blank sheet or the preceding laminated sheet is prepared, a paste for heater insulating layer formation (hereinafter, an insulating paste) is printed in a predetermined pattern to form, on the blank sheet or the preceding laminated sheet, a portion of the heater insulating layer 74 adjacent to the second substrate layer 2 (step S22). The printing may be repeated a plurality of times depending on the thickness of the heater insulating layer 74 to be formed.

Used as the insulating paste for heater insulating layer formation is a paste obtained by mixing an inorganic mixed powder, which is obtained in advance by wet mixing MgO and MgAl₂O₄ as the main components of the heater insulating layer 74 with SiO₂, BaCO₃, and Nb₂O₅ added as the sintering aids, and then grinding and drying the mixture, with a binder component (a dispersant, an organic solvent, a polyvinyl butyral resin, and a nonionic surfactant) dissolved in advance, and adjusting the mixture to have a predetermined viscosity. Mg₄Nb₂O₉, which exists as the main component in the heater insulating layer 74 along with MgO and MgAl₂O₄, is generated in a subsequent firing process by a reaction of the raw powder.

At the time, as a raw powder for each of MgO and MgAl₂O₄, a raw powder having an average particle size of approximately 0.1 μm to 50 μm is used. As a raw powder for Nb₂O₅, a raw powder having an average particle size of approximately 0.1 μm to 20 μm is used. In order for the heater insulating layer 74 to satisfy the above-mentioned main component weight percentage conditions, weight percentages of the raw powders are required to be in the following ranges. The total weight percentage is naturally 100 wt %:

MgO: 39.0 wt % to 61.0 wt %;

MgAl₂O₄: 32.0 wt % to 56.0 wt %;

Nb₂O₅: 0.3 wt % to 10.0 wt %; and

SiO₂ and BaCO₃: 3 wt % or less in total.

By satisfying these requirements, the heater insulating layer 74 that is dense to have a porosity of 4.0% or less, can prevent or reduce diffusion of Pt, and has a coefficient of thermal expansion approximately equal to that of zirconia can be provided. If the above-mentioned requirements are not satisfied, it is difficult to form the heater insulating layer 74 that is dense to be able to prevent or reduce diffusion of Pt and has a coefficient of thermal expansion approximately equal to that of zirconia.

When the insulating paste is printed in the pattern, a paste for heater pattern formation is printed so as to be laminated to form the heater pattern on the pattern of the insulating paste (step S23). In forming the heater pattern, different pastes may be used depending on a target of formation (the heater element 72, the heater leads 72 a, and the resistance detection lead 72 b).

Finally, the insulating paste is printed again in a predetermined pattern to form a portion of the heater insulating layer 74 adjacent to the third substrate layer 3 (step S24). At the time, the insulating paste is printed so that the heater pattern is concealed between a pattern to be formed and the pattern of the insulating paste printed previously. At a location at which the heater pattern does not exist, the pattern in which the insulating paste is printed later is superimposed on the pattern in which the insulating paste is printed previously. The printing may also be repeated a plurality of times depending on the thickness of the heater insulating layer 74 to be formed.

More specifically, the patterns are formed using the insulating paste and the paste for heater pattern formation in view of contraction at the time of firing so that they satisfy in the resulting sensor element 101 conditions other than condition relating to a total length L of the sensor element 101. Patterns of other components, such as various electrodes, are also formed on conditions that targets of formation are formed to have predetermined sizes in the resulting sensor element 101.

After pattern printing on each of the blank sheets, printing of a bonding paste and drying are performed to laminate and bond the green sheets corresponding to the respective layers (step S3).

Then, the green sheets to which an adhesive has been applied are stacked in a predetermined order, and the stacked green sheets are crimped under predetermined temperature and pressure conditions to thereby form a laminated body (step S4). Crimping yields a state in which the heater pattern and the pattern for heater insulating layer formation covering the heater pattern are sandwiched between the green sheet that becomes the second substrate layer 2 after firing and the green sheet that becomes the third substrate layer 3 after firing.

Specifically, crimping is performed by stacking and holding the green sheets as a target of lamination in a predetermined lamination jig (not illustrated) while positioning the green sheets at the sheet holes, and then heating and pressurizing the green sheets together with the lamination jig using a lamination machine, such as a known hydraulic pressing machine. The pressure, temperature, and time for heating and pressurizing depend on a lamination machine to be used, and these conditions may be set appropriately to achieve good lamination.

After the laminated body is obtained as described above, the laminated body is cut out at a plurality of locations to obtain individual units (referred to as element bodies) of the sensor element 101 (step S5). More specifically, the laminated body is cut out in view of contraction at the time of firing so that the resulting sensor element 101 satisfies the sensor element dimensional conditions relating to the total length L.

The cut out element bodies are fired at a firing temperature of approximately 1300° C. to 1500° C. (step S6). As a result, the sensor element 101 that includes the heater insulating layer 74 having a coefficient of thermal expansion of 10.3 to 12.3 (×10⁻⁶/° C.) as the densified layer having a porosity of 4.0% or less is generated.

The sensor element 101 thus obtained is housed in a predetermined housing, and incorporated into a main body (not illustrated) of the gas sensor 100.

As described above, according to the present embodiment, in the planar sensor element including the heater part inside the base part formed of a solid electrolyte, the heater insulating layer is provided as the densified layer to have a coefficient of thermal expansion approximately equal to that of the surrounding solid electrolyte layers. Specifically, the heater insulating layer 74 is provided to have a porosity of 4.0% or less, contain MgO, MgAl₂O₄, and Mg₄Nb₂O₉ as the main components in a weight percentage of 97 wt % to 100 wt % in total, contain each of MgO and MgAl₂O₄ in a weight percentage of 30 wt % to 60 wt %, and contain Mg₄Nb₂O₉ in a weight percentage of 0.5 wt % to 15 wt %. This suitably prevents or reduces diffusion of Pt and the occurrence of cracking caused by the difference in coefficient of thermal expansion between the solid electrolyte layers and the heater insulating layer when the sensor element is used continuously and the temperature cycle including heating during use and cooling after use is repeated. This means that, according to the present embodiment, the planar sensor element has an increased lifetime.

<Modifications>

In the above-mentioned embodiment, description is made by taking, as an example, a case where the sensor element 101 is a planar sensor element having a series two-chamber structure included in the limiting current NOx sensor. However, an embodiment of providing a heater insulating layer so that the heater insulating layer has a porosity of 4.0% or less and a coefficient of thermal expansion approximately equal to that of the surrounding solid electrolyte layers in order to increase a lifetime of a sensor element is applicable not only to a sensor element having a similar structure and detecting another gas species but also to various planar sensor elements in which the sensor part and the heater part are integrally formed.

For example, the embodiment may be applied to a sensor element having three internal spaces and included in a limiting current gas sensor, a sensor element having internal spaces arranged in a different manner from that in the above-mentioned embodiment, or a sensor element included in a mixed potential gas sensor.

Examples

Eight types of sensor elements 101 including the heater insulating layers 74 containing MgO, MgAl₂O₄ (spinel), and Mg₄Nb₂O₉ in different weight percentages while satisfying the main component weight percentage conditions were manufactured as Examples 1 to 8. For each of the sensor elements 101, the porosity and the coefficient of thermal expansion of the heater insulating layer 74 were measured. In addition, a durability test was conducted through continuous heating by the heater part 70 for a long period of time, and the state after the test was evaluated.

The insulating paste used to form the heater insulating layer 74 was manufactured in the following manner. First, powders of MgO and MgAl₂O₄ (spinel), powders of SiO₂, BaCO₃, and Nb₂O₅, IPA as a solvent, and cobbles made of Al₂O₃ were put into a pot mixer having a volume of 1 L, and a mixture obtained by mixing them in the pot mixer at 100 rpm for 24 hours was dried to obtain an inorganic mixed powder. The inorganic mixed powder and the binder component (dispersant, organic solvent, polyvinyl butyral resin, and nonionic surfactant) dissolved by being agitated by an automatic orbital agitator for four minutes were agitated and mixed by the automatic orbital agitator for 15 minutes using the cobbles made of Al₂O₃, thereby to obtain an insulating paste having a disintegration degree of 50 μm or less and a viscosity of 20 Pa·s to 40 Pa·s.

Weight percentages (in wt %) of materials in Examples 1 to 8 are shown in Table 1. In each of Examples, weight percentages of SiO₂ and BaCO₃ were each set to 1.5 wt %, so that the resulting heater insulating layer contained the main components in a weight percentage of 97 wt % in total relative to the sum of the main components and the sub components in a weight percentage of 100 wt %.

TABLE 1 EX- EX- EX- EX- EX- EX- EX- EX- AMPLE AMPLE AMPLE AMPLE AMPLE AMPLE AMPLE AMPLE MATERIALS 1 2 3 4 5 6 7 8 MgO 46.5 53.0 37.0 61.0 42.0 39.0 49.0 59.7 MgAl₂O₄ 46.5 40.0 56.0 32.0 45.0 57.7 38.0 37.0 Nb₂O₅ 4.0 4.0 4.0 4.0 10.0 0.3 10.0 0.3 SiO₂ 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 BaCO₃ 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5

The weight percentages of the main components of the heater insulating layer 74 were specified through Rietveld analysis. Specifically, X-ray diffraction (XRD) measurement (2θ/θ measurement) was conducted in a range in which 2θ was 10° to 80° using a portion corresponding to the heater insulating layer 74 cut out of the sensor element 101 as a test sample, and Rietveld analysis was conducted on an XRD pattern as obtained. As crystal phases, MgO (see Crystal Structure Gallery on the website of the National Institute of Advanced Industrial Science and Technology), MgAl₂O₄ (ICSD 24766), and Mg₄Nb₂O₉ (COD 1526786) were used. TOPAS from Bruker AXS, Inc. was used as analysis software.

The porosity of the heater insulating layer 74 was set to 2.5%. This was achieved by setting a firing temperature at which an element body was fired to obtain each of the sensor elements 101 to 1400° C.

The porosity of the heater insulating layer 74 was specified in the following manner. First, an SEM image (a reflected electronic image at a magnification of 1000 times and a resolution of 1.2 million pixels) of a cross section perpendicular to the longitudinal direction of the sensor element 101 and including the heater element 72 was captured after polishing the cross section so that no particle fell. Two areas of the heater insulating layer 74 in the SEM image of the cross section each having an area of 2000 μm² or more were specified as porosity calculation areas, and values of the porosity were calculated for the respective two areas through image processing. An average value of the obtained two values was set to the porosity in Example or Comparative Example.

The porosity was calculated through image processing using Image-Pro Premier 9.2 (from Nippon Roper K. K.) as software. Specifically, for each of the porosity calculation areas, binarization and mask image generation were performed so that a portion other than pores became a mask area, an area ratio (%) of an area other than the mask area was obtained, and a value of the area ratio was set to the porosity for the porosity calculation area.

The coefficient of thermal expansion was obtained by using the portion corresponding to the heater insulating layer 74 cut out of the sensor element 101 as the test sample and alumina as a reference substance and repeating heating and cooling them from a room temperature to 900° C. at 10° C./min twice by a thermomechanical analyzer (from Rigaku Corporation, TMA8310), and by using a ratio (L₀ (%)) of dimensions to initial dimensions at 40° C. in the second heating and cooling and a ratio (L₁ (%)) of dimensions to the initial dimensions at 900° C. in the second heating and cooling by the following equation: coefficient of thermal expansion (×10⁻⁶/° C.)=10⁴×(L ₁ −L ₀)/(900(° C.)−40(° C.))

The durability test was conducted on condition that heating was performed continuously using the heater element 72 at 900° C. for 2000 hours. With respect to the sensor element 101 after the durability test, whether cracking occurred or not was determined and whether Pt diffused in the heater insulating layer 74 or not was determined based on the SEM image, and besides, the heater resistance values before and after the test were measured, and the resistance increase rate after the test was calculated by substituting the results of measurement into the equation (2).

As Comparative Example 1, the sensor element 101 was manufactured in a similar manner to that in Example 1 except that the firing temperature at which the element body was fired to obtain the sensor element 101 was set to 1280° C.

As Comparative Examples 2 and 3, the sensor elements 101 were manufactured in a similar manner to that in Example 1 except that the weight percentages of MgO and MgAl₂O₄ were out of the main component weight percentage conditions.

Furthermore, as Comparative Example 4, the sensor element 101 was manufactured in a similar manner to that in Example 1 except that the weight percentage of Mg₄Nb₂O₉ fell below the lower limit in the main component weight percentage conditions.

In addition, as Comparative Example 5, the sensor element 101 was manufactured under the same conditions as those in Examples 1 to 8 except that the heater insulating layer 74 was formed using an insulating paste containing, as a main component, alumina (Al₂O₃) suitable to form a porous alumina (Al₂O₃) layer. Alumina used as a material for the insulating paste had an average particle size of 0.5 μm to 1.0 μm and a specific surface area of approximately 70 m²/g to 100 m²/g.

Furthermore, as Comparative Example 6, the sensor element 101 was manufactured under the same conditions as those in Examples 1 to 8 except that the heater insulating layer 74 was formed using an insulating paste containing, as a main component, alumina (Al₂O₃) suitable to form a dense alumina (Al₂O₃) layer. Alumina used as a material for the insulating paste was α-alumina having an average particle size of 0.05 μm to 0.4 μm and a specific surface area of approximately 10 m²/g to 30 m²/g.

Weight percentages (in wt %) of materials in Comparative Examples 1 to 6 are shown in Table 2.

TABLE 2 COMPAR- COMPAR- COMPAR- COMPAR- COMPAR- COMPAR- ATIVE ATIVE ATIVE ATIVE ATIVE ATIVE PARTICLE EX- EX- EX- EX- EX- EX- MATERIALS SIZE (μm) AMPLE 1 AMPLE 2 AMPLE 3 AMPLE 4 AMPLE 5 AMPLE 6 MgO 0.1-50  46.5 33.0 65.0 48.5 — — MgAl₂O₄ 0.1-50  46.5 60.0 28.0 48.5 — — Nb₂O₅ 0.1-20  4.0 4.0 4.0 0.1 — — Al₂O₃ 0.5-1.0 — — — — 97.0 97.0 SiO₂ 0.5-1.0 1.5 1.5 1.5 1.5 1.5 1.5 BaCO₃ 0.05-0.4  1.5 1.5 1.5 1.5 1.5 1.5

For each of the sensor elements 101 according to Comparative Examples 1 to 6, the porosity and the coefficient of thermal expansion of the heater insulating layer 74 were measured, and a durability test was conducted through continuous heating by the heater part 70 for a long period of time, and the state after the test was evaluated, as with the sensor elements according to Examples 1 to 8.

For each of the sensor elements 101 according to Examples 1 to 8, the porosity, weight percentages of the main components, and the coefficient of thermal expansion of the heater insulating layer 74 and various evaluation results are listed in Table 3. For each of the sensor elements 101 according to Comparative Examples 1 to 6, the porosity, weight percentages of the main components, and the coefficient of thermal expansion of the heater insulating layer 74 and various evaluation results are listed in Table 4. In Tables 3 and 4, the heater insulating layer 74 is simply indicated as an “INSULATING LAYER”.

TABLE 3 EX- EX- EX- EX- EX- EX- EX- EX- AMPLE AMPLE AMPLE AMPLE AMPLE AMPLE AMPLE AMPLE 1 2 3 4 5 6 7 8 INSULATING POROSITY (%) 2.5 2.5 2.5 2.5 2.5 25 25 25 LAYER MAIN MgO 37.0 51.0 31.0 60.0 37.0 37.0 45.0 59.5 COMPONENT MgAl₂O₄ 54.0 40.0 60.0 31.0 45.0 59.5 37.0 37.0 COMPOSITION Mg₄Nb₂O₉ 6.0 60 6.0 6.0 15.0 0.5 15.0 0.5 wt (%) COEFFICIENT OF THERMAL 10.5 11.3 10.3 12.3 10.3 10.4 10.8 11.5 EXPANSION (×10⁻⁶/° C.) STATE AFTER OCCURRENCE NOT NOT NOT NOT NOT NOT NOT NOT DURABILITY TEST OF CRACKING IDEN- IDEN- IDEN- IDEN- IDEN- IDEN- IDEN- IDEN- TIFIED TIFIED TIFIED TIFIED TIFIED TIFIED TIFIED TIFIED DIFFUSION NOT NOT NOT NO NOT NOT NOT NOT OF Pt IDEN- IDEN- IDEN- IDEN- IDEN- IDEN- IDEN- IDEN- TIFIED TIFIED TIFIED TIFIED TIFIED TIFIED TIFIED TIFIED RESISTANCE 1.0 0.8 1.2 1.1 1.2 1.1 1.1 1.1 INCREASE RATE (%)

TABLE 4 COMPAR- COMPAR- COMPAR- COMPAR- COMPAR- COMPAR- ATIVE ATIVE ATIVE ATIVE ATIVE ATIVE EX- EX- EX- EX- EX- EX- AMPLE 1 AMPLE 2 AMPLE 3 AMPLE 4 AMPLE 5 AMPLE 6 INSULATING POROSITY (%) 4.3 2.5 2.5 4.6 35.1 4.0 LAYER MAIN MgO 37.0 28.0 64.0 45.0 (POROUS (DENSE COMPONENT MgAl₂O₄ 54.0 63.0 27.0 51.7 Al₂O₃) Al₂O₃) COMPOSITION Mg₄Nb₂O₉ 6.0 6.0 60 0.3 wt (%) COEFFICIENT OF THERMAL 10.5 10.1 12.8 10.6 8.5 8.6 EXPANSION (×10⁻⁶/° C.) STATE AFTER OCCURRENCE NOT IDEN- IDEN- NOT NOT IDEN- DURABILITY TEST OF CRACKING IDEN- TIFIED TIFIED IDEN- IDEN- TIFIED TIFIED TIFIED TIFIED DIFFUSION IDEN- IDEN- IDEN- IDEN- IDEN- IDEN- OF Pt TIFIED TIFIED TIFIED TIFIED TIFIED TIFIED RESISTANCE 2.4 2.4 2.5 2.6 2.6 2.5 INCREASE RATE (%)

Comparing Examples 1 to 8 and Comparative Examples 1 to 4, in Examples 1 to 8 in each of which the heater insulating layer 74 had a porosity of 2.5%, which was less than 4.0%, and a coefficient of thermal expansion having a value in a range of 10.3 to 12.3 (×10⁻⁶/° C.), neither the occurrence of cracking nor diffusion of Pt was identified after the durability test. The resistance increase rate was less than 2%.

In contrast, in the sensor elements 101 according to Comparative Examples 1 and 4, the heater insulating layers 74 had coefficients of thermal expansion of 10.5 (×10⁻⁶/° C.) and 10.6 (×10⁻⁶/° C.), which were approximately equal to that in Example 1, but had porosities of 4.3% and 4.6%, which exceeded 4.0%. For each of these sensor elements 101, the occurrence of cracking was not identified but diffusion of Pt was identified after the durability test. The resistance increase rates were 2.4% and 2.6%, which exceeded 2%.

Taking the results of Comparative Examples 1 and 4 and the results of Examples 1 to 8 together, it can be said that diffusion of Pt is not prevented or reduced when the porosity of the heater insulating layer 74 exceeds 4.0%.

In each of the sensor element 101 according to Comparative Example 2 in which the coefficient of thermal expansion was only 10.1 (×10⁻⁶/° C.) and the sensor element 101 according to Comparative Example 3 in which the coefficient of thermal expansion had a large value of 12.8 (×10⁻⁶/° C.), although the porosity of the heater insulating layer 74 fell below 4.0%, both the occurrence of cracking and diffusion of Pt were identified after the durability test. The resistance increase rate exceeded 2%.

The reason why cracking occurred in each of the sensor elements 101 according to Comparative Examples 2 and 3 is considered that, due to a large difference in coefficient of thermal expansion between the heater insulating layer 74 and the surrounding solid electrolyte layers (in particular, the second substrate layer 2 and the third substrate layer 3), stress caused by the difference in coefficient of thermal expansion was not sufficiently relaxed. The reason why Pt diffused despite the heater insulating layer 74 formed as the densified layer having a small porosity is considered that diffusion was caused through cracks as formed.

Taking the results of Comparative Examples 2 and 3 and the results of Examples 1 to 8 and Comparative Examples 1 and 4 together, providing the heater insulating layer 74 as the densified layer having a coefficient of thermal expansion having a value in a range of 10.3 to 12.3 (×10⁻⁶/° C.) as in the above-mentioned embodiment, specifically, providing the heater insulating layer 74 so that the heater insulating layer 74 has a porosity of 4.0% or less and satisfies the main component weight percentage conditions, is effective in preventing or reducing both the occurrence of cracking and diffusion of Pt in the sensor element 101.

In the sensor element 101 according to Comparative Example 5, the value of the coefficient of thermal expansion was only 8.5 (×10⁻⁶/° C.) while the porosity of the heater insulating layer 74 was 35.1%, which exceeded 4.0% significantly. The occurrence of cracking was not observed after the durability test while diffusion of Pt was identified after the durability test. The resistance increase rate was more than 2.0%.

The results of Comparative Example 5 mean that the heater insulating layer 74 provided as a porous layer does not have the function of preventing or reducing diffusion of Pt but has the function of relaxing stress to the surrounding solid electrolyte layers (in particular, the second substrate layer 2 and the third substrate layer 3). Also considering the results of Examples 1 to 8 and Comparative Example 1, it can be said that the results of Comparative Example 5 are results indirectly indicating that, in each of the sensor elements 101 according to Examples 1 to 8, the occurrence of cracking is prevented or reduced by a mechanism different from that in the sensor element 101 according to Comparative Example 5.

Furthermore, in the sensor element 101 according to Comparative Example 6, the heater insulating layer 74 had a porosity of 4.0% but had a coefficient of thermal expansion of only 8.6 (×10⁻⁶/° C.). Both diffusion of Pt and the occurrence of cracking were identified after the durability test. The resistance increase rate was more than 2.0%.

The results of Comparative Example 6 indicate that, even if the heater insulating layer 74 is simply provided as the densified layer, the occurrence of cracking cannot be prevented or reduced, and thus diffusion of Pt cannot be prevented or reduced when the coefficient of thermal expansion is small.

While the invention has been shown and described in detail, the foregoing description is in all aspects illustrative and not restrictive. It is therefore understood that numerous modifications and variations can be devised without departing from the scope of the invention. 

What is claimed is:
 1. A planar sensor element included in a gas sensor for detecting a predetermined gas component in a measurement gas, said sensor element comprising: a base part that is formed of zirconia as an oxygen-ion conductive solid electrolyte; and a heater part that heats said sensor element, wherein said heater part includes: a heater element that generates heat by being externally powered, and contains Pt; an insulating layer that covers said heater element; and a heater electrode that is located on a main surface of said sensor element to be exposed, and is electrically connected to said heater element, a portion of said heater part other than said heater electrode is buried in said base part, said insulating layer at least contains MgO, MgAl₂O₄, and Mg₄Nb₂O₉ in a weight percentage of 97 wt % to 100 wt % in total, MgO being contained in a weight percentage of 30 wt % to 60 wt %, MgAl₂O₄ being contained in a weight percentage of 30 wt % to 60 wt %, Mg₄Nb₂O₉ being contained in a weight percentage of 0.5 wt % to 15 wt %, and said insulating layer has a porosity of 4% or less.
 2. The sensor element according to claim 1, wherein said insulating layer has a coefficient of thermal expansion at least at 900° C. of 10.3 to 12.3 (×10⁻⁶/° C.).
 3. The sensor element according to claim 2, further comprising: a gas inlet that is located at one end portion of said base part; at least one internal space that is located inside said base part, and communicates with said gas inlet under a predetermined diffusion resistance; and at least one electrochemical pump cell that includes an outer pump electrode located on an outer surface of said base part, an inner pump electrode located to face said at least one internal space, and said solid electrolyte located between said outer pump electrode and said inner pump electrode, and pumps in and out oxygen between said at least one internal space and an outside, wherein said portion of said heater part other than said heater electrode is located, below a gas distribution part that extends from said gas inlet to said at least one internal space, along a direction in which said gas distribution part extends while being separated from said gas distribution part by said solid electrolyte.
 4. The sensor element according to claim 1, further comprising: a gas inlet that is located at one end portion of said base part; at least one internal space that is located inside said base part, and communicates with said gas inlet under a predetermined diffusion resistance; and at least one electrochemical pump cell that includes an outer pump electrode located on an outer surface of said base part, an inner pump electrode located to face said at least one internal space, and said solid electrolyte located between said outer pump electrode and said inner pump electrode, and pumps in and out oxygen between said at least one internal space and an outside, wherein said portion of said heater part other than said heater electrode is located, below a gas distribution part that extends from said gas inlet to said at least one internal space, along a direction in which said gas distribution part extends while being separated from said gas distribution part by said solid electrolyte. 